Process and apparatus for separating air



Feb. 25, 1958 A L 2,824,428

PROCESS AND APPARATUS FOR SEPARATING AIR Original Filed Aug. 16, 1954 v 2 Shets-Sheet 1 &

INVENTOR N v EDWARD F'. YENDALL BY aha 4 03% /ATTORNEY F Feb. 25, 1958 E. F. YENDALL 2,324,428

' PROCESS AND APPARATUS FOR SEPARATING AIR Original Filed Aug. 16, 1954 A 2 Sheets-Sheet 2 INAAIENTOR EDWARD F.' YENDALL BY fiJ/dM a ATTORNEY United tates P tent "19 Claims. (Cl.62i23) v This invention relates to a r process offandapparatus for separating rair -'irito products including an ar on- I *enrichedvproduct, and -more particularlyfor-improving the yield of such argonx'pr'oduct. v p I v *C ustomarilycommercial'argon is recovered fromair during the two-stage rectification of army drawingefr an argon-containing vaporfrom anintermediate level of the secondstage, whichvap'or is further 'ct'ified in an auxiliary rectificationto produce 'a crud'e'a'argon product 'icontaining a small" residue of nitrogen and some "oxygen.-

=By further treatment the 'oxygen" 'cont'ent may be 'elir'ninated and the nitrogen residue c'onte'nt reduced-to analmost immeasurableamount. Usuallythe'oxygen prod- 1 net isrecoveredas a compressed gas or as-a liquid for commercial use, and while "in some cases somepure nitrogen product may be recovered from the first-stage 'of rectificatiom'm'ost of the nitrogen constitutes thefwaste efiiuent returned to the air "afterth'e refrigeration therein is recovered.

By the customary processes less than halfof tlie toial. amount of argon in 'the air processe'd -is recoveredintlie argon-containing product. As the industrial demand 'for argon increases it becomes important to increase the --yild. Since nominal increases of yield may 'notjbe suflicientto satisfy the demand, itbec'or'ns economically practical to sacrifice either partor allof the "oxygen: production to increase the yield of *ar'gon'or even to sep rate 'air torecoverar'gon asthe'sole product while returningthe oxygen with theefiluentto atmosphere atter recovery of'the refrigeration. 'lt'is'therefore aprincipal'objectof the presenrin' e tion to provide an'improved process and system for separating 'air into its products including anai"gon'prbduct with "substantially increased yield recoveryof theirgon. inthe air processed. V

A' further object of the invention is to provide an iiiiproved process and system for "separation-of air pm duce a crudeargonproduct as the sole recoveredpro'diict. of'theseparation. p

1A furtherpbject is to provide animpro'vemenfin the: two stage' separation of air whereby increased yields of" argon: are obtainable, whereby the system can be'operated to *increase the yield of argon at the sacrifice of more or'less'of' the oxygen product production'according'totherelative demands for the argon and oxygen productsrand whereby'a substantially pure nitrogen product can also be produced' when required. I

Other objects and advantages of the invention will*become" apparent as the description proceeds in connection with the accompanying drawings, in which:

Fig. lisa schematic flow diagram of anexe'inplafy system forthe-separation of-air according to the'inve'ntion particularly suitable for obtaining an argon product as the sole recovered product'or with 'also'aminor amount of oxygen product, and

Fig. 2 is a schematic 'fiow diagram according-to tlie' invention of a system for separation of air to provide bOtha variable amount of liquid oxygen product "andan 'ir'rac'tion'bybubbling up through perforatedcon't'act" 2,824,428 Patented Feb. 25, 1958 optional small amount of nitrogen product in addition "to 'animproved' yield of argon product.

According to the invention, it has been discovered that in a two-stage rectiiicationsystemfor air separation that,

5 whenthe mainrectification is operated so'that' a minimum ambitntofar'gon remains'with theliquid oxygen product *6lltd at the base zone-'of-the' mai'nrectification, that substantial lossof' argon with the elfiuent'frornthcjtop er the' -inain rectification column 'can be reducedbyf't'lie "ster 'of ali-quidcontaining very little argon tothe V tificati'o'n column and'preferablyabbve *sevral rectification "trays provided in the column a ove the feed point -of the nitrogen-rich reflux liquid to the eal-limb. Airfliec'rt iife'liquid-for this purpose has bee n 15 found to be a portion ofor the entire liquid-oxygeniprodproducers tlie tdp of the main rectifyingcolumn. "Such oxygen not oiily"balancesthesym odiiction 'ofsome' pure fnit'rdgenbutit al'so siipp'lemehts the 'reiiiix reiiuii'einer'it' in their in c'ohiiri'n to reve'nt loss of arg onwith"the"columnkfiifientantl rases theylield of'ar'gon. o x g 7 ring riow to Fig. "1; air iscompressed in afilr'liocompressor system 10 and passed, to a main heat ex- "changer systein B "by a-condu'it 11. Therfi'resihre is at f and 125 p. s. i. g. *The heat" exchaiigei"' inay be an ofthetypes'eiistdmar ily employed fbfefieeting eatxarige betweenair'andefiluent. "Preferablyithe typeemfployedrriay be recuperative or regenerative "operatedtd'reatmvamtsture aiidfc'arbon dioxide from the incomingair for evaporation 'into'theoutgoing efliuenrgas. Cold umam-tars are shewn as an example 'of s ich type of"mai'n h'e'at exchangers. The coldaclc'umulators' 12 and 13arepfro- *vided at their'upper 'war m ends w'ith a"piping"systm"'14 "containing 'ever sing valves" to-which thepipe 11 eonneets. The air, during one new -period of the "reversing 1 cycle, fflows down through one accumulator l2 fil-ledwithflpre- "cooled h ea bs'orbing material of'large surface area. The moisture and carbon dioxide are-depositedon' themater'ial'while' the airj gives up-"itsheat to themateri al'and comes out at" the"cold end' substantially 'free', of moisture and 'carbon dioxide and cooled" to a temperature close to its condensation "temperature at the existing pressiire. h lowerreversin valve "fem I5 through'a pipe 16 intothe" lower nd'of ascrubber 'lchan lber li v I g V V Y 1 Iriscrubbe'r chamber 17 the air is s'crubb'e'cl' with al'quid G8 and the-cleaned vaper"pcirtismfiews througfh" co diiit from the upper part of chamber 17 .intdthe loiver part of a higher pressure rectifying'c olurnn 20. In t-he :co'lu'mn 20 the rising vapors" contact''deiscendirig nitrogen- .rich liquid -and'are partially condensed to formacrude oxygenIiquid-atthe base of the columnFZl). A main Q. rectifiyingcolumnar is superimposed on thedaighvprefis sure column 20 and coupled in heat exchange therewith by .a vapor condenser 22 surrounded by liquid oxygen collected in the base zone 23 of the main column. The vapor from the column 20 enters and is condensed in the condenser 22 to provide reflux liquid forthe column 20 (and to provide a liquid rich in nitrogen which is collected on a shelf 24 under the condenser 22. The

liquid fraction from the base of the high pressure column 20 is conducted by a conduit 25 to an intermediate level feed point 26 of the main column, there being a throttling valve 27 interposed therein. The nitrogen'rich liquid collected on the shelf 24 is transferred by a conduit 28 to a higher level feed point 29 of the main column 21 above the feed point 26. The transfer conduit 28 has interposed therein a throttling valve 30 and preferably also a subcooling heat exchanger 31 which is cooled by efiluent of the top of the main column passed to heat exchanger passage 32 by conduit 33. I

When the refrigeration production in the system is limited to that required for recovery of all products in warm gaseous state, the amount of liquid fraction used for scrubber liquid may be so limited that it is desirable to employ a recirculation system for impurity elimination of the type shown in Patent 2,572,933 of R. W. Houvener. The scrubber liquid fraction which has collected residual impurities is withdrawn from overflow cup 35 of the scrubber chamber 17 byconduit 36a to the inlet of a liquid pump 36 from the discharge of which a conduit 36b to impurity-removing devices 37 which may be filters or a combination of filter elements and adsorbent-containing traps, and then returned by a conduit 38 ,to the scrubber preferably to a point below at least part of the plates 18. If refrigeration production is great enough, for example when producing some liquid oxygen product, excess scrubber liquid fraction can be passed to the intermediate level feed point 26 of the main rectification either by a separate transfer line or by a conduit 38a connecting to conduit :and containing a control valve 38!).

The scrubber liquid fraction is preferably produced by condensing a diverted portion of the scrubbed air vapor so that condensing surfaces may remain free of impurities. To this end .a conduit 39 diverts air vapor to a condenser 40 surrounded by a heat exchange chamber 4]. The air condensed by the condenser 40 is collected in a chamber 42 under it and then passed by conduit 43 joining conduit 38 into the scrubber 17. The heat exchange chamber 41 receives its coolant in the form of etlluent gas through conduit 44 connecting heat exchange passage 32 therewith. The partly warmed eflluent gas is conducted from heat exchange chamber 41 :through a conduit 45 to the reversing system 15 for passage upward through the cold accumulator 13 during a period of the reversing cycle and from the upper end of the cold accumulator 13 through reversing valve system 14 to the eflluent outlet 46.

Alternatively the scrubber may be positioned in the bottom of the column 20 to employ the crude oxygen as scrubber liquid. in which case the impurity-eliminating devices 37 would be interposed in the transfer line 25.

For producing the low temperature refrigeration requirement of the system an unliquefied portion of scrubbed .air is preferably work-expanded and in order that the work expansion may be efficient and result in an expanded vapor which is close to a dry saturated condition, the vapor to be expanded is pre-warmed' by suitable means, for example, a portion of the scrubbed air is diverted from below the condenser 40 through a conduit 48, passed through one of a pair of heat exchangers 49 and 50 in heat exchange with a portion of the incoming air in heat exchange passages 51 and.52, which air is diverted from an intermediate portion of the cold accumulators 12 and 13 through conduit 53. The air portion cooled by such heat exchange is conducted from the passage 51 or 52 through conduit 54 connected to Tense-12s .4 conduit 16. The pro-warmed portion of air is then conducted from heat exchange passage 49 or 50 through .conduit 55 to the nozzles of a turbo-expander 56.

From the expander 56 a conduit 57 connecting to :an intermediate level of the main rectification column conducts the cold expanded air vapor to the column so :that argon contained therein can be washed out. The refrigeration thus applied to the main column in part passes down the column with refluxing liquids and in part passes up the column with the rising vapors eventually forming the effluent gas in conduit 33 so that at .least part of the expander refrigeration is employed in the heat exchange chamber 41 for liquefaction of scrubbed air vapor to form scrubber liquid.

Alternatively, the system may be operated as a socalled excess air cycle or partial excess air cycle by increasing the amount of air through the expander 56 and -providing a by-pass connection 58 connecting with the efiiuent line 44. Valves 59 and 60 are interposed in connections 57 and 58 to regulate the flow of expanded air to the columnand to the effluent line 44 as required. Up to about 15% of the air can be passed as expanded .air to the column.

The point in the main column where the argon content of the rising vapors is relatively high and where the nitrogen content in such vapors is very low or substantially a minimum is a desirable level for withdrawal of argon-containing vapor for further processing. This level 61 is several trays below the intermediate feed point 26. A conduit 62 conducts the argon-containing vapor to the lower portion of an auxiliary rectifying column 63. In the column 63 the rising vapors meet a descending flow of reflux liquid which washes the oxygen content to the lower end of the column 63 from which such oxygen-rich liquid is passed by conduit 64 back to the main rectifying column. The column 63 may be placed at such a level that the liquid flows by gravity toward the main column. If desired, however, a liquid pump can be interposed in the conduit 64 so that the base of the column 63 can be positioned at a lower level.

The reflux liquid for the. auxiliary column 63 is prefer- .ably provided by positioning a reflux condenser 65 at its upper end through which the rising vapors of the column must pass. The condenser 65 is preferably refrigerated by a cold fluid from the system, for example, a portion of the transfer liquid of conduit 25 may be diverted through a conduit 66 and passed into a boiling chamber 67 surrounding the condenser 65, a throttling valve 68 being interposed in the conduit 66. The liquid portion that does not flash into vapor after passage through throttling valve 68 forms, a cold refrigeration liquid in chamber 67 which boils to condense a portion of the vapors passing through condenser 65. The vapor from boiling chamber 67 is conducted by conduit 69 to the main column 21 at a point close to the intermediate level 26. The crude argon product is withdrawn,preferably from the upper part of the condenser 65 through a conduit 70. Such crude argon product may be further processed in another rectifying column in the customary manner for elimination of residual nitrogen orit may be collected as a Warm crude argon gas to be sent to an argon refinery, not shown. The latter procedure is illustrated in Fig. 1 wherein the crude argon product through conduit 70 is passed to heat exchangefcoils 71 and 72 which are in heat exchange relation with the heat exchange material in the cold accumulators 12 and 13. The conduit 70 connects to the cold end of the passages 71 and 72 and the warm ends of such passages connect to a conduit 73 which delivers the crude argon product to receiving means.

For washing "argon out of the rising effluent vapor in the main rectifying column there is preferably added some gas and'liquid contact means in the for'm'of several rectification trays at the upper end of the column between the feed point 29 and the top of the column,

*conduit78to the upper en'dofthemaincolhmnfil. "The rlate'of-flow -to-"the pump may be controlled byvarying thespeedof the pump byayariable speed "drive and by providing a by-pass'from conduit 78 to conduit 77 controlled by valve 79. *A desired amount of liquid oxygen product could'bewithdrawnby regulating a valve'80 in "abranch conn'ection .81 of conduit -78.

Theoperation-of the system isbelievd clear from the 'abovedescription. The expander 56 can be provided with-"several expansion nozzles so thatthe amount of "-ainexpandedcan' be varied-withinlimits, andfor greater variation twoor more-expanders'can'beconnected to the inlet pipe -55=and outlet-pipe 57., When the system is in balanced operation with the valve 60 closed and {valve59 open-so that all the turbine exhaust passes into ---the main rectifying column, it is found that the argon i eeovered will be about 90% ofthe'total argon available.

Thepower cost of operating thesyst'ern'for argon recove'ry'asthesoleproduct isalso found quite reasonable. When'the turbine exhaust is diverted to the efiluent pipe 44. with-valve-59 closed'and valve 60 open-and the proportion of air which is passed through the expander is about 15%, then the recovery of the argon in the product is-found to be about 83% of the argon available. "If some liquid oxygen is withdrawn as 'aproduct thenirigeration loss contained in the-amount of-liquidoxygen =withdrawn must be providedby expansion of 'an extra amountof air. Such excess air may be provided by operating additionalexpansion nozzles in the turbine or by placing-' in-service an additional expansion turbine, hind-such excess expanded air-is passedthrough the con- 1ie'ctiorr58 to the eflluent pipe 44. A further control for 'columnoperation is obtained by an oxygen-richyapor discharge connection 82a from column-zone 23 directly to theellluent line 44 and controlled by a valve 82.

Referring now to Fig. 2, there is illustrated a' rectifying ccihtmn-portion of-a system for producing liquid oxygen a'rid sorne liquid nitrogen product as well asa crude argon product. The portion of such system brooding and preparing the I air for rectification and I forproviding the required low temperature refrigeration may be of a customary-type,- for example, similar to thatshown in Patent No. 2,547,177 issuedto G. E. Simpson, April 3, 1951.

-In Fig. 2 the air whichhas been cooled, freed of moisture, andpartially liquefied at about 75 p.- s. i. g. enters through-pipe'116 into'scrubber chamber 117 where the "vapor portion is'scrubbed with the liquidportion. The impurity-containing liquid portion passes through 'a pipe 136:and impurity-removingdevices-137 and pipe 138 to an interrnedi-ate level 126 of the main rectifying column 121. The vapor portion of the air passes through pipe 119 to -thelower part of the high pressure stagerecti- 'fy ing column120. The liquid crude'oxygen collecting at -the lower part of column 120 passes through transfer pipe 125*and' throttling valve 127 to theintermediate level 126 of the main column. The liquid nitrogen collected atshelf 124 at the upper part of the column '120and below the condenser122 is conducted by pipe 128 and throughheatexchanger pass 131 and throttling valve 13 0 the higher level 129 of the main rectifying column. The'efi luent from the top of'the main column leaves through-conduit133 to heat exchanger pass 132 from -'whicl1 it is conducted by conduit 144 to the mainheat exchanger system for cooling incoming air.

A desired portion of liquid nitrogen to form a liquid nitrogen product may be withdrawn through a branch 85 of the conduit 128, the withdrawal being controlled by 'a valve 86. The point of connection between the conduit 85 and conduit 128 may be on either side of the heatexchanger passage 131. However,-i-f it is desired to .withdraw subcooled liquid nitrogen the connection must be *between '-the"exchanger 131 and valve- 130, the "sub- "cooling being"desired to prevent excessive'amountsof the nitrogen-product from flashing into 'vap or when its "pressure is reducedthrough "valve86. A liquid oxygen product maybewithdr'awnfrom 'the boiling chamber zone 123 of" the 'main' column through 'a connection 81 having a control valvef'SO. 'The'liquid e'x gen product which" is not withdrawn as 'product through valve is recirculated through conduit 177to"the liquid oxygen pump 176 and conduiti178 to' the upper end of the m'ain rectifying column above upp'er rectifyii-Ig trays175.

Thexarg omcontaihing:vapor is withdrawn from a point r or thernain column jbelow'finterr'riedia'te level 126 and passed" into the auxiliary column 163 for rectification therein to produce an oxygen-rich bottom "liquid which is withdrawn through'condiiit164'arid' passed'bac kto the main rectification. A reflux condensed-1650f the auxiliary column is refrigerat'ed'by a portion of the crude oxygen transfer liquid diverted through .a conduit 166 and throttling valve 168 to the boiling chamber 167 around the condenser165. The vapor is-conducted from the boiling chamber through"conduit1'69 to the main column' just past theinte'r'rriediate level 126. The crude argon product is withdrawn by conduit 170 from the upper part of the reflux condenser 165.

In the system according to Fig. 2; sufficient refrigera- -tion is produced and pa s'sed'to the'rectifying system in the form of liquid that all theoxygen product of separation could be withdrawn as liquid oxygen' an'd some of the liquid nitrogen can be withdrawn as product. "However, as more liquid nitrogen is' withdrawn less is available for reflux liquid forthe main column but it ispossible, ac-

cording to the invention, to balance'the's'ystem by withdrawing less liquid oxygen at'valve'80 'as'product and transferring more by the pump 176 to 'the upper end of the main column. [This extra refrigeration and;reflux supplied to the'top'of the main'column'al'so improves the'washing down of argon so that the-argon recovery is improved.

The system provides fiexibilityfor meeting variable demands for the threeproducts. Fore'xa'rnple, the system can be operated to provide substantially no liquid nitrogen product and a maximum of liquid oxygen'prodnot, in which case the recovery of argon would be about 45% of the argon available. If, however, less'liquid oxygen product is required the argon recovery can be substantially improved byipumping some oxygen to the top of the main column. When an increased amount of liquid nitrogen product is required, the liquid oxygen production can be further reduced and the recirculation of oxygen to the top of the main column correspondingly increased with further improvement of the argonrecovery as well as of the nitrogen product recovery. The system of Fig. 2'can'also be operated to makel no oxygen product so that all the oxygen is recirculated as reflux to the top of the main column, thus increasing the argon recovery to the maximum available. I

This application is a continuation of myapplication Serial No. 449,963, filed August 16, 1954, now aban- 'doned.

What is claimed is:

1. A process for separating air bylow temperature rectification into products including an argon-enriched product which comprises subjecting air to, cooling and liquefying heat exchanges including a rectification at a higher rectificationpressure to provide liquid feeds for a main rectification at lower pressure, said feeds including an oxygen-enriched liquid containing a substantial part of the argon and a nitrogen-rich liquid containing a minor amount of the argon; feeding the oxygen-enriched liquid to an intermediate feed point level of said main rectification after expansion to said lower pressure; feeding the nitrogen-rich liquid 'to a higher "feed pointlevel of saidliriain rectifi'eationafter expansion to said tewer feed point and passing it to an auxiliary rectification; refrigerating the eflluent of said auxiliary rectification to partially liquefy same and provide a reflux liquid for said auxiliary rectification and an argon-enriched product; boiling the oxygen-rich liquid collected at the lower zone of said main rectification to produce vapor for said main rectification and to reduce the argon content of such oxygen-rich liquid to an immaterial amount; and feeding at least a portion of the unvaporized oxygenrich liquid from the lower zone of the main rectification to the top zone thereof above said higher feed point level to wash down a substantial amount of the argon contained in the nitrogen vapor above said higher feed point whereby an improved yield of argon is recoveredin said argonenriched product.

2. A process for separating air by low temperature rectification according to claim 1 in which said refrigeration of the efiluentof said auxiliary rectification is effected by heat exchange between the effluent and a portion of a liquid of the system and the resulting vapor of such heat exchange is passed to the main rectification.

3. A process for separating air by low temperature rectification according to claim 1 in which the entire unvaporized oxygen-rich liquid is passed to the upper zone of the main rectification and the argon containing crude product and the top etfiuent are the sole products of the separation.

4. A process for separating air by low temperature rectification according to claim 1 in which a part of the unvaporized oxygen-rich liquid is passed to the upper zone of the main rectification and a desired remainder of the unvaporized oxygen-rich liquid is withdrawn as a product of the separation.

5. A process for separating air by low temperature rectification according to claim 1 in which a part of the unvaporized oxygen-rich liquid is passed to the upper zone of the main rectification, a desired remainder of the oxygen-rich product is withdrawn as a product of the separation, and a portion of said nitrogen-rich fraction is withdrawn as another product of the separation.

6. In a process for separating air by low temperature rectification into products including an argon-enriched product in which liquids made from air are treated in a main rectification stage having an oxygen-rich liquid reboiler at its lower end, an oxygen-enriched liquid fraction containing argon is introduced at an intermediate feed point, a nitrogen-rich liquid fraction'containing a small amount of argon is introduced at a higher feed point, and from which an argon containing crude product is withdrawn at a level below said intermediate feed point, the improvement for increasing the yield of argon which comprises passing at least a portion of the unvaporized oxygen-rich liquid collected in said reboiler to an upper zone of said main rectification above saidhigher feed point to wash down argon from the nitrogen yapors above said higher feed point.

7. A process for separating air by low temperature rectification according to claim 6 in which the entire unvaporized oxygen-rich liquid is passed to the upper zone of the main rectification and the argon containing crude product and the top eflluent are the sole products of the separation. I

8. A process for separating air by low temperature rectification according to claim 6 in which apart of the unvaporized oxygen-rich liquid is passed to the upper zone of the main rectification anda desired remainder of the unvaporized oxygen-rich. liquid is withdrawn as a product of the separation.

9. A process forseparating air by low temperature rectification accordingto claim 6 in which a part of the unvaporized oxygen-rich liquid is passed to the upper zone of the main rectification, a desired remainder of the oxygen-rich liquid is withdrawn as a product of the sepa- Cit ration, and a portion of said nitrogen-rich fraction is withdrawn as another product of the separation.

10. Process for the low temperature separation of air to recover an argon product which process comprises cooling air at a condensation pressure between 60 and p. s. i. g. by heat exchange withefliuent gas to provide an air stream close to condensation temperature corresponding to the pressure and substantially free of moisture and carbon dioxide; scrubbing the air stream with a liquid fraction to collect residual impurities in the liquid fraction so that the gaseous part of the stream is substantially free of impurities; efiecting partial liquefaction of said gaseous part to produce said liquid fraction; eliminating impurities from said liquid fraction; passing the cleaned liquid fraction to a main rectification at lower pressure at an intermediate level thereof; diverting a portion of said gaseous part and Warming such portion by heat exchange with air to a temperature such that upon subsequent work-expansion a substantially dry saturated vapor will result; expanding such warmed portion with the production of external work; effecting heat exchanges for using at least part of the low temperature refrigeration of said work-expansion for effecting at least part of said liquefaction; effecting heat exchange between the remainder of said gaseous part of the stream and oxygenrich liquid collected at the base zone of the main rectification to produce a nitrogen-rich liquid; passing a sub stantial part of said nitrogen-rich liquid to said main rectification at a higher feed level above said intermediate level; passing at least part of the oxygen-rich liquid from the base zone of the main rectification to an upper level of the main rectification above said higher level; withdrawing an argon-containing vapor of improved argon yield from a level of said main rectification below said intermediate level; and subjecting such argon-containing vapor to auxiliary rectification to separate oxygen-rich liquid for return to themain rectification and produce a crude argon product.

ll. In a process for the low temperature separation of air in which the air is subjected to two stages of rectification to produce at least some nitrogen product taken from the higher pressure stage, an efiluent gas from the top of the lower pressure stage and at least one other product, an oxygen-rich liquid product from the lower zone of the lower pressure stage, the improvement for meeting varying demands for said products which comprises increasing the withdrawal rate of said nitrogen product and reducing the withdrawal rate of said oxygen-rich product while correspondingly withdrawing from the lower zone of said lower pressure stage sufficient liquid rich in oxygen for passage to the top of said lower pressure stage as reflux liquid to balance the refrigeration requirements and compensate the reduction of reflux liquid created by said increased withdrawal of said nitrogen product.

12. In a process for the low temperature separation of air in which the air is subjected to two stages of rectification into an overhead top effluent gas, an oxygen product, and an argon-containing product removed from an intermediate part of the second stage and in which reflux feeds for the second stage in the form of crude oxygen and liquid nitrogen are fed to the second stage at successively higher levels the improvement for increasing the recovery of argon which comprises adjusting the liquid-vapor reflux ratio in the upper zone of the second stage to increase the oxygen content of the reflux liquid for washing down the argon by supplementing the liquid feed to the higherzone with a liquid high in oxygen content and low inargon content, the eflluent thereby containing more oxygen and less argon.

' 13. In a process for the low temperature separation of air in which the air is subjected to two stages of rectification into an overhead top efiluent gas, an oxygen prodnet; and an argon-containing product removed from an intermediate part of the second stage and in which reflux feeds for the second stage in the form of crude oxygen and liquid nitrogen are fed to the second stage at successively higher levels the improvement for increasing the recovery of argon which comprises adjusting the liquid vapor reflux ratio in the upper zone of the second stage to increase said oxygen content of the reflux liquid for washing down the argon by passing a portion of an oxygen-rich liquid of small argon content in the system to the higher zone of the rectificatoin at a level at least as high as the nitrogen reflux feed level.

14. In a system for the separation of air by low temperature rectification including higher pressure and lower pressure main rectification column chambers associated so that vapor of the higher pressure column is condensed in a heat exchanger by heat exchange with oxygen-rich liquid collected in the base zone of the main column chamber, means for feeding oxygen-containing liquid from the base of the higher pressure column to an intermediate level of the main column, means for collecting a portion of nitrogen-rich liquid produced by said heat exchanger and transferring same to said main column at a higher level above said intermediate level, an auxiliary rectifying column with a reflux condenser at its upper end, means for feeding argon-containing vapor from the main column at a level below said intermediate level to said auxiliary column, means for feeding liquid collected at the base of said auxiliary column to said main column, and means for withdrawing an argon-enriched product from said reflux condenser of the auxiliary column; the improvement which comprises means providing in said main rectifying column additional gas and liquid contact means between the upper end of the main column and said higher level; and means for passing oxygen-rich liquid from the base zone of said main column to the upper end of said main column above said additional gas and liquid contact means.

15. A system for the separation of air according to claim 14 which includes means for diverting, expanding, and passing a portion of a liquid of the system in heat exchange with said refiux condenser of the auxiliary column and for passing vapor of such heat exchange to said main column.

16. A system for the separation of air according to 10 claim 14 which includes means for also withdrawing a liquid oxygen product from the base zone of said main column.

17. A system for the separation of air according to claim 14 which includes means for withdrawing a nitrogen product from the upper end of the higher pressure column, and means for subcooling such nitrogen product by heat exchange with efliuent gas of the main rectification before discharge to receiving means.

18. In a process for the low temperature separation of air inv which the air is subjected to two stages of rectification in the second stage of which is produced an overhead top effluent gas, a bottom oxygen liquid which is reboiled to produce vapor for product withdrawal and for the second stage rectification, and an argon-containing product removed from an intermediate part thereof and in which reflux feeds for the second stage in the form of crude oxygen and liquid nitrogen are fed to the second stage at successively higher levels, the improvement for increasnig the recovery of argon which comprises adjusting the liquid-vapor reflux ratio in the upper zone of said second stage to increase the oxygen content of the reflux liquid for washing down the argon by supplementing the liquid feed to the higher zone with a liquid high in oxygen content and low in argon content and by regulating the rate of oxygen vapor product withdrawal from said second stage.

19. A process for separating air by low temperature rectification according to claim 6 in which part of the oxygen vapor produced in said reboiler is passed to the upper zone of the main rectification and part is withdrawn as a product of the separation, and which includes the step of regulating the rate of oxygen vapor product withdrawal from said main rectification to control the liquid-vapor reflux ratio therein.

References Cited in the file of this patent UNITED STATES PATENTS 1,880,981 .Pollitzer et a1. Oct. 4, 1932 2,433,508 Dennis Dec. 30, 1947 2,547,177 Simpson Apr. 3, 1951 

1. A PROCESS FOR SEPARATING AIR BY LOW TEMPERATURE RECTIFICATION INTO PRODUCTS INCLUDING AN ARGON-ENRICHED PRODUCT WHICH COMPRISES SUBJECTING AIR TO COOLING AND LIQUEFYING HEAT EXCHANGES INCLUDING A RECTIFICATION AT A HIGHER RECTIFICATION PRESSURE TO PROVIDE LIQUID FEEDS FOR A MAIN RECTIFICATION AT A LOWER PRESSURE, SAID FEEDS INCLUDING AN OXYGEN-ENRICHED LIQUID CONTAINING A SUBSTANTIAL PART OF THE ARGON AND A NITROGEN-RICH LIQUID CONTAINING A MINOR AMOUNT OF THE ARGON, FEEDING THE OXYGEN-ENRICHED LIQUID TO AN INTERMEDIATE FEED POINT LEVEL OF SAID MAIN RECTIFICATION AFTER EXPANSION TO SAID LOWER PRESSURE, FEEDING THE NITROGEN-RICH LIQUID TO A HIGHER FEED POINT LEVEL OF SAID MAIN RECTIFICATION AFTER EXPANSION TO SAID LOWER PRESSURE, WITHDRAWING AN ARGON-CONTAINING VAPOR FROM A LEVEL OF SAID MAIN RECTIFICATION BELOW SAID INTERMEDIATE FEED POINT AND PASSING IT TO AN AUXILIARY RECTIFICATIONS, 